This invention relates to an acoustic tool break detection system and method to automatically adjust the cutting vibration signal to a desired average during a machining operation while detecting a broken tool by monitoring the cutting signal.
A machine tool monitor (MTM) which interprets vibration signals generated by the interaction of a cutting tool with a workpiece, and serves as a tool touch and tool break detector, is described in several commonly assigned copending applications and in published papers such as "Automatic Tool Touch and Breakage Detection in Turning", S. R. Hayashi et al, ICMA, Hanover, West Germany, Sept. 18-19, 1985. The MTM is comprised of a vibration sensor such as an accelerometer, an analog channel with amplification and signal preprocessing, an analog-to-digital converter, and a microprocessor with associated digital hardware. The accelerometer typically has a broadband response which the analog channel bandpass filter restricts. The gain stage keeps signals within the dynamic range of the system, and an energy detector is provided consisting of a full wave rectifier and low pass anti-aliasing filter. In one analog preprocessor the accelerometer output signal is conditioned to a 0 to 10 volt dc 500 Hz signal for input to the digital processor. An algorithm analyzes this signal to determine if a tool break has occurred by looking for a rapid sustained change in a signal level. It is desirable to maintain the average A/D converter input level at approximately one volt for dynamic range considerations. This means that the gain of the analog amplifier must be set every time a level change is expected due to a process cutting change such as depth of cut or different tool-workpiece combinations.
As set forth in copending application Ser. No. 744,083, filed June 12, 1985 now U.S. Pat. No. 4,724,524, C. E. Thomas et al, "Vibration-Sensing Tool Break and Touch Detector Optimized for Machining Conditions", the analog channel gain control is comprised of a multiplying digital-to-analog converter used as a variable attenuator whose gain is adjusted by setting its binary inputs via a signal from the digital hardware. The desired gain may be calculated in the MTM from machining parameters transferred from the part program, and alternatively the desired gain is programmed as part of the parts program. The gain change can be done by the part programmer through part programming additions or manually by switch settings; eliminating this operation unloads the programmer from extra code generation. There is also a problem in that a cutting process does not necessarily produce a constant signal level. The signal can change over the full 10 volt range during a process, for a gain setting that produces a one volt average for the majority of the operation. It is desirable to minimize this variation without affecting the rapid sustained level changes indicative of a tool break.
U.S. Pat. No. 4,514,797 is relevant and discloses a software-controlled automatic gain control (AGC) for a worn cutting tool detector.